1. Field of the Invention
The present invention relates to an optical information recording medium having a plurality of recording layers to/from which information is recorded/reproduced by irradiation with laser light or the like, and an initializing method and an initialization device for initializing the optical information recording medium.
2. Related Background Art
Optical information recording media have drawn attention as large-capacity and high-density memories, and so-called erasable types of the same, which are rewritable, are being developed now. The erasable optical information recording media include a medium that includes as a recording layer a thin film that exhibits a phase transition between the amorphous state and the crystalline state, to which information is recorded and erased by thermal energy generated by the irradiation with laser light.
As the phase-change materials for the foregoing recording layers, films made of alloys composed of Ge, Sb, Te, In, etc. as principal components are known, for instance, a GeSbTe alloy. Information is recorded by forming recording marks, which are obtained by causing the recording layer partially to make a transition into amorphous phase. Generally, information is erased by crystallizing the recording marks in many cases. The transition to the amorphous state is achieved by heating the recording layer to above a melting point of the same and thereafter cooling the same. On the other hand, the crystallization is achieved by heating the recording layer to a temperature above a crystallization temperature and below the melting point. Moreover, the recording layer is formed generally by sputtering, but a thin film of the aforementioned phase-change material formed by sputtering exhibits an amorphous state in most cases. Therefore, it is necessary to make the recording layer assume a crystalline state prior to recording information. This process is called “initialization”.
A conventional initialization device for performing the foregoing initialization has one optical head equipped with one light source and one objective lens, and initializes a desired area of the recording layer by irradiating the recording layer of the recording medium with a light beam emitted from the optical head while moving the optical head in a predetermined direction.
On the other hand, recently, the volume of information handled increases as the processing ability of information devices improves. Therefore, an increased-capacity recording medium capable of recording/reproducing at an increased speed has been demanded. To achieve such an increased capacity and an increased speed, a multilayer recording medium has been proposed, which is configured so as to include a plurality of recording layers, so that information can be recorded/reproduced to/from each of the recording layers from one side of the medium (e.g. JP 9(1997)-91700A).
However, the initializing method according to JP9(1997)-91700A initializes the recording layer closer to the beam irradiation side first, which results in a drawback in that the recording layers tend to have irregularities. More specifically, portions initialized have less transmittance of light because of crystallization, which hinders the recording layer farther from the foregoing side from being initialized uniformly.
Furthermore, in the case where the foregoing multilayer recording medium is initialized by a conventional initialization device, only one recording layer is initialized at once. Therefore, it is necessary to repeat the initializing operation a number of times equal to the number of recording layers, which results in a drawback in that a time required for the initialization increases significantly. Besides, whereas a short-wavelength light beam is employed for the recording/reproduction of a recording medium to increase the recording density of the medium, a long-wavelength light beam is employed for the initialization of the recording medium to increase a light intensity of the beam. Therefore, a light beam with a wavelength different from the optical design wavelength for the recording medium is used for the initialization, which destabilizes the focus servo that is for adjusting the focus position precisely to the recording layer. This incurs initialization irregularities in the recording layer, thereby deteriorating the performance of the recording medium. For a multilayer recording medium in particular, it is necessary to bring the focal position precisely to a specific recording layer among the plurality of recording layers, thereby making the foregoing problem more severe.